This invention relates in general to flight control in aerospace platforms during the post-boost phase of operation and, in particular, to a modulation technique for providing pulse-width modulation for operating the thruster valves used to provide attitude and velocity control of the aerospace platforms.
For the deployment of packages from an aerospace platform during the post-boost phase of a mission profile, it is necessary to provide attitude and velocity control of the platform in response to flight control commands. This attitude and velocity control is provided by a plurality of integrated valve assemblies connected to a solid propellant gas generator. Each integrated valve assembly (IVA) has a pair of oppositely aligned high thrust nozzles which are oriented parallel to the missile longitudinal axis and a pair of oppositely aligned low thrust nozzles which are oriented perpendicular to the missile longitudinal axis. Each nozzle can be independently opened or closed by a command signal from a flight control package. The opposing valve pairs are operated in a pulse-width-modulation mode in order to present a nearly constant expulsion area to the gas generator.
One method of pulse-width modulation, known as constant flow modulation, is illustrated in FIG. 1a which show the thrust command time history of a valve pair during one modulation period T. Level +1 indicates that one of the two valves is commanded "on", level -1.0 indicated that the second valve is commanded "on", and level 0 indicates a valve "off" command. With a zero average thrust command (0 percent modulation), the valve commands appear as the solid curve 10. Modulating the command to provide non-zero thrust is achieved by varying the transition point where one valve is switched off and the other valve is switched on as indicated by dashed lines 12 representing 25 percent modulation. Constant flow modulation has the advantage of having a linear average-thrust versus command characteristic near zero thrust command. However, it has the disadvantage that near zero thrust command it causes a large exitation of the bending modes.
A second modulation method, known as null flow modulation, does not appreciably excite bending modes. However, near zero thrust command, null flow modulation has a non-linear transfer characteristic which increases the difficulty of deployment with accuracy. As represented by the solid lines 14 and 16 in FIG. 1b, in null flow modulation, zero average thrust (0 percent modulation) is achieved with both valves continuously "on". When a small average thrust is desired, one of the valves must be turned "off" and "on" over very short period as indicated by dashed lines 18 and 20, respectively. The inability of real valves to achieve this cycling for periods less than approximately 30 milliseconds causes the non-linear characteristic near zero thrust command.